1. Field of the Invention
Apparatus and methods consistent with the present invention relate to a silica-based single core optical fiber, a silica-based multi core optical fiber, and a fabrication method for the same, which control an auto fluorescence phenomenon in a visible wavelengths region occurred when transmitting light of the light region (about 400 nm to about 800 nm). In particular, exemplary embodiments of the present invention relate to a silica-based single core optical fiber, a silica-based multi core optical fiber, and a fabrication method for the same, which may be used in confocal fluorescent imaging, fluorescence detection, luminescence detection, a spectroscopy, and the like, by using an optical fiber.
2. Description of the Related Art
Currently, diagnosis and analysis using fluorescence analysis, luminescence detection, and a spectral analysis using light in light regions including fluorescent diagnostics, are conducted, and in particular the wavelength used has become short with the technical progress of semiconductor lasers.
In recent years, in various medical fields such as those related to gastroenterology, pulmonary disease, or cardiovascular disease, endoscopes are employed for direct observation of surfaces of tissues or as auxiliary apparatuses for medicine. Further, fluorescence diagnostics carried out in combination with an endoscope attracts great deal of attention in recent years.
A tissue irradiated and excited by excitation light emits a fluorescent light having a characteristic spectrum. When the tissue has a lesion such as a tumor or a cancer, the tissue emits a particular fluorescent light having a spectrum different from the normal characteristic spectrum. The fluorescence diagnostics is a diagnostic method utilizing such a characteristic to discern a tissue having a lesion from a normal tissue.
As this diagnostic method does not require collection of tissues from a patient's body, patients are released from physical burden. This is one of the many advantages of this method. A fluorescent diagnostics apparatus suitable for this diagnostic method is disclosed in Japanese Publication H08-224240.
Furthermore, the confocal imaging method in particular using multi core optical fiber attracts attention among the fluorescent diagnostics. According to this method, an observation part which has shown fluorescence is observable with clear and high resolution, and tissue which is in a depth (under the surface) of several 10 to several 100 micrometers from the part can be analyzed.
A confocal fluorescent imaging apparatus using multi core optical fiber is disclosed in the published Japanese translation JP 2005-532883 of the International Application PCT/FR2003/2196.
In this confocal fluorescent imaging apparatus, toward one of a plurality of cores of the multi core optical fiber, an excitation light beam is incident from the incidence edge side and excitation light emitted from an emitting end of multi core optical fiber is illuminated to biomedical tissue of an object. The excitation light beam is an excitation light beam which has wavelengths, such as 405 nm, 488 nm, or 635 nm, for example (outputs are about 10 mW to about 30 mW, and beam diameters are about 1 micrometer to about several micrometers).
As a result, from the biomedical tissue, auto fluorescence according to a state of the tissue occurs in a region where a wavelength is longer than a wavelength of the excitation light. This fluorescence is transmitted to an incidence edge through the same core with the excitation light, and after excitation light is separated, predetermined information (spectral intensity and shape) is obtained, a predetermined process is performed and auto fluorescence has is obtained.
Such operation is performed by scanning the whole fiber end with speed of per second 12 frames, and a two-dimensional image is obtained by obtaining a confocal image for every pixel. Since the illumination of excitation light to the biomedical tissue and the transmission of fluorescence are performed by the same core, fluorescence from sources other than a focal plane is removed by effect of spatial filtering, and, as a result, confocal characteristics are achieved.
When the inventors of the present invention experimented with improving the multi core optical fiber used for the above confocal fluorescent imaging methods for the purpose of quality improvement of an image obtained by the confocal fluorescent imaging method, the inventors found out the following problems. The experiment will be explained including procedures.
FIG. 1 shows a schematic configuration diagram of the spectrum measurement apparatus 100 applied to auto fluorescence spectrum measurement of a related art multi core optical fiber.
As shown in FIG. 1, the spectrum measurement apparatus 100 applied to auto fluorescence spectrum measurement of a related art multi core optical fiber mainly includes a light source 102 which emits a laser beam with a wavelength of 488 nm as excitation light, an optical lens 104 for condensing the laser beam into a beam from the light source 102 about 2 to 3 micrometers in diameter, a multi core optical fiber 106 in which the beam passed through the optical lens 104 is incident, an objective lens 108 provided in an emitting end of the multi core optical fiber 106, and a CCD 110 optically combined with the objective lens 108.
The spectrum measuring device 100 further includes a dichroic filter 112 allowing laser light from the light source 102 to pass through and reflecting light reflected by the objective lens 108 and passing through the multi-core optical fiber 106 and the optical lens 104 in this order, and a spectrum analyzer 114 to receive the light reflected by the dichroic filter 112 (light reflected by the objective lens 108) and carry out analysis of the light.
The spectrum measurement apparatus 100 further includes an XYZ stage 116 which adjusts a position of the multi core optical fiber so that between the optical lens 104 and the multi core optical fiber 106 may be connected optically, and a reflective filter 118 for noise reduction.
FIG. 2 shows a spectrum, in which an exemplary embodiment has a relatively sharp induced Raman scattering light wavelength around 515 nm and a broad auto fluorescence peak in the wavelength range of about 520 nm to 750 nm produced by the light source 102 emitting 488 nm laser light (single mode, for example, 22 mW), collecting the light by the optical lens 104 to have the light incident into one of cores of the multi-core optical fiber 106, and carrying out analysis of the light reflected by the objective lens 108 and returned back.
Moreover, in an exemplary embodiment where the wavelength of the excitation light is 440 nm, as shown in FIG. 3, induced Raman scattering light centering on near the wavelength of 460 nm and a broad auto fluorescence in a range of wavelengths of about 500 nm to about 720 nm is found.
Also in an exemplary embodiment where the wavelength of the excitation light is 635 nm, a longer wavelength range has been observed that includes an auto fluorescence ranging from the excitation light wavelength to about 200 nm longer than the excitation light wavelength.
Thus, wavelengths of fluorescence from tissue being imaged are observed which are in a range of as auto fluorescence from the fiber. This causes poor accuracy of fluorescent diagnostics and reduces a signal-to noise (S/N) ratio.
It is guessed that same luminous phenomenon may become a problem not only for a fluorescent imaging method using the above multi core optical fiber but also in fluorescence detection, luminescence detection, and spectroscopy using a single core fiber.